Shutdown but Restart (Toshiba Satellite A200)

On a Toshiba Satellite A200-1FN PSAECE-02600SIT with Windows 7 32 bit fresh installation or "full" installation (win update, drivers, sw) when I press SHUTODWN to shutdown the laptop, the system is go normaly in shutdown. But, after 1-2 seconds he is go back on... It seems that I chose RESTART instead of SHUTDOWN.

I also tried to update the BIOS with the version 1.80 available for Vista (for Windows 7 since there is no update).

I found that this happens only if the power supply is attached. If the laptop only runs on battery, turns off and remains off without rebooting.

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